Starfish facts for kids
Quick facts for kids Starfish |
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| Fromia monilis | |
Scientific classification  |
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| Kingdom: | Animalia |
| Phylum: | Echinodermata |
| Superclass: | Asterozoa |
| Class: | Asteroidea De Blainville, 1830 |
| Child taxa and orders | |
† Calliasterellidae |
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Starfish, or sea stars, are Echinoderms of the Class Asteroidea.
All live in the ocean, on the sea floor. Many starfish live in deep water, others in shallow water. Some live in the intertidal zone, between low and high tide. They have five or more arms and can be quite large. The Sunflower sea star (Pycnopodia helianthoides) is the largest: fully grown, its arm-span is about a metre. This is a bit larger than the famous Crown-of-thorns starfish, which lives on coral polyps.
Although starfish are invertebrates, they do have a kind of skeleton. The bodies of starfish are composed of calcium carbonate plates, known as 'ossicles'. These form the endoskeleton, which takes on a variety of forms such as spines and granules.
They have a primitive nervous system, but not a brain. They also do not have blood, instead they use sea water to pump things around their bodies.
There are over 1500 different species of sea star. Most sea stars are predators. They eat mussels, clams, and other bivalves. Occasionally, they catch small fish.
Contents
Symmetry
Most starfish have five arms, called rays that come out from a center circle (disk). If a starfish has more than five rays, it will often have rays in multiples of five; there could be 10, 15, 20, or even 30 rays on one starfish. This is called pentameral (five-fold) symmetry.
Skeleton
The degree of protection offered by the skeleton differs between species. If the skeleton becomes more rigid, it offers better protection against predators. However, this will tend to limit its feeding alternatives. Great flexibility is required for the process of external digestion used by many species to eat bivalves. The less well armoured species may adopt a life in places where they are not so open to predation. The heavily armoured ones can tough it out in the most competitive habitats:
- "A heavily calcified sturdy skeleton capable of resisting both large and small predators has evolved in many tropical sea stars... a very firm body wall... still permits a degree of flexibility of the body and arms".
Feeding
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Senses
The movement of starfish is guided by their senses of touch and sight. There are five 'eyes', light-sensitive cushions, one at the end of each arm. These and the tube feet are connected to nerve fibres, so these animals are more complex than might appear.
Tube feet
Although starfish started off as filter-feeders, they evolved to become major predators of shell-fish (the brachiopods and bivalves). They can also eat small crustacea and fish. Their tube feet developed suckers, perhaps originally to improve movement. Later, they were used to open shell-fish.
- "Suckered tube feet may not have been present in any Palaeozoic sea star".
Feeding methods
The shells of brachiopods and bivalves are held together by strong muscles. What the starfish does is clamp hold of them on either side with its tube feet, and apply a steady pull. The starfish, with its muscles and hydraulic system, can pull for much longer than any bivalve muscle can withstand. Apparently, ten minutes are usually enough to open the shell a bit. Then the starfish slips its stomach inside the shell. The stomach can get through a slot as narrow as 0.1mm. The starfish then dissolves the mollusc where it lives, absorbing the nutrients. This digestion process takes much longer than opening the shell, perhaps a couple of days.
Some species swallow the shell whole, and dissolve the contents inside their stomach, then push out the shell afterwards.p45
The ability of starfish to eat brachipods and bivalves developed in the Mesozoic, especially in the Jurassic and Cretaceous. This was part of the Mesozoic marine revolution, which transformed the sea-floor fauna. Weakly defended and static shellfish disappeared, and more heavily armoured or more mobile shellfish flourished.
Regeneration
Starfish, like many sea creatures, are able to regenerate (grow back) parts of their bodies. Starfish are better at regeneration than most other creatures. Not only can a new ray grow when a ray is torn off, but if the torn off ray has even a small piece of the central disk still attached, a whole new starfish can grow from the one ray.p35
Because starfish like to eat clams and oysters, fishermen who gather shellfish have tried for years to get rid of them. To kill the starfish, fishermen would catch them, slice them right in half, and throw them back in the ocean. However, because starfish can grow back parts of their bodies, they were actually increasing the number of starfish.
Human relations
In research
Starfish are deuterostomes, closely related, together with all other echinoderms, to chordates, and are used in reproductive and developmental studies. Female starfish produce large numbers of oocytes that are easily isolated; these can be stored in a pre-meiosis phase and stimulated to complete division by the use of 1-methyladenine. Starfish oocytes are well suited for this research as they are large and easy to handle, transparent, simple to maintain in sea water at room temperature, and they develop rapidly. Asterina pectinifera, used as a model organism for this purpose, is resilient and easy to breed and maintain in the laboratory.
Another area of research is the ability of starfish to regenerate lost body parts. The stem cells of adult humans are incapable of much differentiation and understanding the regrowth, repair and cloning processes in starfish may have implications for human medicine.
Starfish also have an unusual ability to expel foreign objects from their bodies, which makes them difficult to tag for research tracking purposes.
In legend and culture
An aboriginal Australian fable retold by the Welsh school headmaster William Jenkyn Thomas (1870–1959) tells how some animals needed a canoe to cross the ocean. Whale had one but refused to lend it, so Starfish kept him busy, telling him stories and grooming him to remove parasites, while the others stole the canoe. When Whale realized the trick he beat Starfish ragged, which is how Starfish still is today.
In 1900, the scholar Edward Tregear documented The Creation Song, which he describes as "an ancient prayer for the dedication of a high chief" of Hawaii. Among the "uncreated gods" described early in the song are the male Kumulipo ("Creation") and the female Poele, both born in the night, a coral insect, the earthworm, and the starfish.
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Georg Eberhard Rumpf's 1705 The Ambonese Curiosity Cabinet describes the tropical varieties of Stella Marina or Bintang Laut, "Sea Star", in Latin and Malay respectively, known in the waters around Ambon. He writes that the Histoire des Antilles reports that when the sea stars "see thunder storms approaching, [they] grab hold of many small stones with their little legs, looking to ... hold themselves down as if with anchors".
Starfish is the title of novels by Peter Watts and Jennie Orbell, and in 2012, Alice Addison wrote a non-fiction book titled Starfish - A year in the life of bereavement and depression. The Starfish and the Spider is a 2006 business management book by Ori Brafman and Rod Beckstrom; its title alludes to the ability of the starfish to regenerate itself because of its decentralized nervous system, and the book suggests ways that a decentralized organisation may flourish.
In the Nickelodeon animated television series SpongeBob SquarePants, the eponymous character's best friend is a dim-witted starfish, Patrick Star.
As food
Starfish are widespread in the oceans, but are only occasionally used as food. There may be good reason for this: the bodies of numerous species are dominated by bony ossicles, and the body wall of many species contains saponins, which have an unpleasant taste, and others contain tetrodotoxins which are poisonous. Some species that prey on bivalve molluscs can transmit paralytic shellfish poisoning. Georg Eberhard Rumpf found few starfish being used for food in the Indonesian archipelago, other than as bait in fish traps, but on the island of "Huamobel" [sic] the people cut them up, squeeze out the "black blood" and cook them with sour tamarind leaves; after resting the pieces for a day or two, they remove the outer skin and cook them in coconut milk. Starfish are sometimes eaten in China, Japan and in Micronesia.
As collectables
Starfish are in some cases taken from their habitat and sold to tourists as souvenirs, ornaments, curios or for display in aquariums. In particular, Oreaster reticulatus, with its easily accessed habitat and conspicuous coloration, is widely collected in the Caribbean. In the early to mid 20th century, this species was common along the coasts of the West Indies, but collection and trade have severely reduced its numbers. In the State of Florida, O. reticulatus is listed as endangered and its collection is illegal. Nevertheless, it is still sold throughout its range and beyond. A similar phenomenon exists in the Indo-Pacific for species such as Protoreaster nodosus.
In industry and military history
With its multiple arms, the starfish provides a popular metaphor for computer networks, companies and software tools. It is also the name of a seabed imaging system and company.
Starfish has repeatedly been chosen as a name in military history. Three ships of the Royal Navy have borne the name HMS Starfish: an A-class destroyer launched in 1894; an R-class destroyer launched in 1916; and an S-class submarine launched in 1933 and lost in 1940. In World War II, Starfish sites were large-scale night-time decoys created during The Blitz to simulate burning British cities. Starfish Prime was a high-altitude nuclear test conducted by the United States on 9 July 1962.
Images for kids
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Luidia maculata, a seven armed starfish
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Astropecten aranciacus ossicles
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Pedicellariae and retracted papulae among the spines of Acanthaster planci
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Pedicellaria and papulae of Asterias forbesi
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Arm tip of Leptasterias polaris showing tube feet and eyespot
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Three kinds of bilaterally symmetric starfish larvae (from left to right) scaphularia larva, bipinnaria larva, brachiolaria larva, all of Asterias sp. Painted by Ernst Haeckel
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"Comet" of Linckia guildingi, showing starfish body regrowing from a single arm
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Sunflower seastar regenerating missing arms
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A Circeaster pullus starfish everting its stomach to feed on coral
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Pisaster ochraceus consuming a mussel in central California
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American herring gull feeding on a starfish
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Starfish fossil, Riedaster reicheli, from the Plattenkalk Upper Jurassic limestone, Solnhofen
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Ray fragment (oral surface; ambulacrum) of goniasterid asteroid; Zichor Formation (Coniacian, Upper Cretaceous), southern Israel.
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A large 18-armed member of Brisingida
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Common starfish, a member of Forcipulatida
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Magnificent star, a member of Paxillosida
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Red-knobbed starfish, a member of Valvatida
See also
In Spanish: Estrellas de mar para niños
